Dew point calculator methodology using the Magnus equation
The calculator uses the exponential vapor-pressure relationship documented in National Weather Service meteorological calculator material. Saturation vapor pressure is estimated from temperature using 611 pascals, coefficient 7.5, and temperature constant 237.3°C.
Actual vapor pressure is saturation vapor pressure multiplied by relative humidity. Inverse forms of the same relationship solve dew point, relative humidity, or air temperature without iterative network requests.
es(T) = 611 × 10^(7.5T / (237.3 + T))Heat index calculator methodology using the NWS regression
Heat index begins with the simpler Steadman screening equation. When the screened apparent temperature reaches about 80°F, the calculator applies the Rothfusz regression and the National Weather Service low-humidity or high-humidity adjustment when its defined range applies.
The regression expects Fahrenheit and relative humidity as a percentage. Results are converted to Celsius and Kelvin only after the full Fahrenheit calculation is complete.
HI = -42.379 + 2.04901523T + 10.14333127RH − 0.22475541TRH − …Temperature unit conversion and calculator rounding rules
Temperature inputs are converted to Celsius before the vapor-pressure equations run. Heat index inputs are converted to Fahrenheit for the NWS procedure, then the final apparent temperature is converted to Celsius and Kelvin. Relative humidity remains a percentage throughout.
Calculations retain JavaScript floating-point precision internally, while the interface rounds results to a useful number of decimal places. The displayed precision makes values readable and repeatable; it does not claim that the original observation or sensor was equally precise.
Dew point, humidity, and heat index input validation limits
Displayed temperatures are rounded for readability while calculations retain JavaScript floating-point precision. Water-vapor concentration outputs assume standard sea-level pressure of 101,325 pascals. A station at another pressure will have a different volume or weight concentration.
The interface rejects humidity at or below zero, humidity above 100%, temperatures at or below absolute zero, and a dew point above air temperature in the relative-humidity mode. Equations are estimates and are not a substitute for calibrated observations.
How dew point and heat index calculator results are tested
Automated tests cover temperature conversion, forward and inverse moisture calculations, invalid atmospheric combinations, and heat-index branches. Reversible examples are checked in both directions: a calculated dew point should reconstruct the original RH within the expected rounding tolerance.
Boundary checks include 100% relative humidity, where air temperature and dew point must match, and the NWS screening transition used before the full Rothfusz regression. Published source examples and independent calculations are used as reference points when formulas change.
- Celsius, Fahrenheit, and Kelvin conversion checks
- Dew point, relative humidity, and inverse air-temperature round trips
- Humidity range and dew-point relationship validation
- NWS heat-index screening, regression, adjustment, and risk categories
Primary NWS sources for dew point and heat index formulas
These sources define the equations and meteorological context used by this site. They do not endorse dewpointcalculator.org.